Vertex Group from part of Material
Assigning weight

Add Vertex Weight modifier, create empty vertex group, enable group add and tweak few sliders to get it work.



Selecting vertices

Add Mask modifier and select same group as in Vertex Weight modifier. Also enable all display options to see modifiers working in edit mode (inside green box on screenshot)



Next, you enter edit mode and select everything. Press A twice to toggle selection. It is mandatory to select all NOT by pressing A, use selection tools instead (like border B or circle C) with wireframe view  enabled Z to speed up things a bit. Once you have select everything, assign it to new vertex group.



To check if you done it right, disable viewport visibility of modifiers.